U.S. patent application number 14/005804 was filed with the patent office on 2014-01-16 for high dynamic range rf power monitor.
This patent application is currently assigned to The Johns Hopkins University. The applicant listed for this patent is Paul A. Bottomley, William Edelstein, Abdel-Monem M. El-Sharkawy, Di Qian. Invention is credited to Paul A. Bottomley, William Edelstein, Abdel-Monem M. El-Sharkawy, Di Qian.
Application Number | 20140015547 14/005804 |
Document ID | / |
Family ID | 46880046 |
Filed Date | 2014-01-16 |
United States Patent
Application |
20140015547 |
Kind Code |
A1 |
Bottomley; Paul A. ; et
al. |
January 16, 2014 |
HIGH DYNAMIC RANGE RF POWER MONITOR
Abstract
A device with at least one channel for measuring high dynamic
range, radio frequency (RF) power levels over broad-ranging duty
cycles includes a power sensor circuit comprising at least one
logarithmic amplifier; at least one directional RF coupler
electrically connected to the at least one power sensor; at least
one RF attenuator electrically connected to the at least one RF
coupler; and at least one sampling circuit electrically connected
to the at least one RF attenuator and the at least one RF coupler.
The at least one sampling circuit performs analog-to-digital
conversion of electrical signals received to provide digitals
signals for measuring the RF power level in the at least one
channel.
Inventors: |
Bottomley; Paul A.;
(Baltimore, MD) ; Edelstein; William; (Baltimore,
MD) ; El-Sharkawy; Abdel-Monem M.; (Baltimore,
MD) ; Qian; Di; (Baltimore, MD) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bottomley; Paul A.
Edelstein; William
El-Sharkawy; Abdel-Monem M.
Qian; Di |
Baltimore
Baltimore
Baltimore
Baltimore |
MD
MD
MD
MD |
US
US
US
US |
|
|
Assignee: |
The Johns Hopkins
University
Baltimore
MD
|
Family ID: |
46880046 |
Appl. No.: |
14/005804 |
Filed: |
March 22, 2012 |
PCT Filed: |
March 22, 2012 |
PCT NO: |
PCT/US12/30173 |
371 Date: |
September 17, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61466194 |
Mar 22, 2011 |
|
|
|
Current U.S.
Class: |
324/647 |
Current CPC
Class: |
G01R 27/32 20130101;
A61N 1/00 20130101; G01R 21/00 20130101; G01R 33/288 20130101; G01R
1/00 20130101; G01R 21/10 20130101; G01R 33/3614 20130101; G06F
2101/00 20130101; G06F 1/00 20130101 |
Class at
Publication: |
324/647 |
International
Class: |
G01R 27/32 20060101
G01R027/32 |
Goverment Interests
[0002] This invention was made with U.S. Government support of
Grant Nos. R01 EB07829, awarded by National Institutes of Health
(NIH). The U.S. Government has certain rights in this invention.
Claims
1. A device with at least one channel for measuring high dynamic
range, radio frequency (RF) power levels over broad-ranging duty
cycles, comprising: a power sensor circuit comprising at least one
logarithmic amplifier; at least one directional RF coupler
electrically connected to said at least one power sensor; at least
one RF attenuator electrically connected to said at least one RF
coupler; and at least one sampling circuit electrically connected
to said at least one RF attenuator and said at least one RF
coupler, wherein said at least one sampling circuit performs
analog-to-digital conversion of electrical signals received to
provide digitals signals for measuring the RF power level in the at
least one channel.
2. The device of claim 1, further comprising a computer configured
to communicate with said at least one sampling circuit to receive
said digital signals.
3. The device of claim 2, further comprising a display configured
to communicate with said computer.
4. The device of claim 3, wherein said computer is further
configured to calculate measured peak power from said digital
signals in real time and said display is configured to communicate
with said computer to provide real-time display of said measured
peak power.
5. The device of claim 4, wherein said computer is further
configured to calculate measured average power from said digital
signals in real time from said at least one channel, and said
display is configured to communicate with said computer to provide
real-time display of said measured average power from said at least
one channel.
6. The device of claim 1, wherein the at least one channel is more
than one channel, wherein each channel comprises a power sensor
circuit comprising a logarithmic amplifier connected to a
directional RF coupler electrically connected to a RF attenuator
electrically connected to at least one sampling circuit, and
wherein each said sampling circuit is electrically also connected
to each RF attenuator to provide multichannel RF measurements.
7. The device of claim 6, wherein the at least one channel is four
channels.
8. The device of claim 1, wherein said high dynamic range, low duty
cycle RF power levels are produced by a magnetic resonance imaging
(MRI) system and said device is MRI compatible.
9. The device of claim 8, wherein said device is adapted for
monitoring RF exposure of a subject in said MRI system, wherein
said MRI system operates in a frequency range of 1 MHz to 400
MHz.
10. The device of claim 9, further comprising a Q-hybrid
transducer.
11. A method of monitoring radio frequency (RF) exposure proximate
an RF emitting device, comprising connecting an RF monitoring
system according to claim 1 to an RF power source of said RF
emitting device.
12. The method of claim 11, wherein said RF emitting device is a
magnetic resonance imaging (MRI) system.
13. The method of claim 11, wherein said RF power source is an RF
coil of an MRI system.
14. The method of claim 11, wherein said RF emitting device is at
least one of a radar system or a telecommunications system, or a
medical RF diathermy system, or a RF ablation system.
Description
CROSS-REFERENCE OF RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional
Application No. 61/466,194 filed Mar. 22, 2011, the entire contents
of which are hereby incorporated by reference.
BACKGROUND
[0003] 1. Field of Invention
[0004] The field of the currently claimed embodiments of this
invention relates to high dynamic range (DR) radio frequency (RF)
power monitoring devices and methods.
[0005] 2. Discussion of Related Art
[0006] Accurate knowledge of the RF specific absorption rate (SAR)
in the body during magnetic resonance imaging (MRI) scans is
important for patient safety and compliance with limits mandated by
the Food and Drug Administration (FDA) in the USA.sup.1 and the
International Electro-technical Commission (IEC) in Europe..sup.2
In addition to ensuring safe operation and regulatory compliance,
accurate power monitoring can avoid restrictions on clinical MRI
sequences arising from incorrect estimation of the delivered power.
Accurate knowledge of delivered power is essential for testing the
MRI safety of peripheral, implanted and interventional devices at
defined RF exposure levels..sup.3-6
[0007] RF safety concerns initially arose with the introduction of
higher-field 1.5 Tesla (T) whole-body MRI scanners and the
recognition that SAR increases approximately with the square of MRI
frequency or field-strength when other MRI sequence parameters are
kept constant..sup.7-9 The recent emergence of clinical 3 T
scanners and experimental body systems operating at 7 T and
higher,.sup.10 in which SAR could potentially increase 4- to more
than 20-fold compared to 1.5 T, only exacerbates concerns about
safety and how to ensure compliance with SAR
guidelines..sup.1,2
[0008] In clinical MRI scanners, SAR monitoring for safety and
regulatory compliance is generally carried out by scanner software
and hardware that is largely proprietary, with "scanner SAR" values
typically logged for each study. These systems prohibit or
terminate scanning based on predictions of body SAR relying on
internal measures, modeling, and prior characterization or assumed
properties of the MRI transmit coil. Electromagnetic modeling with
knowledge of the input power .sup.11-13 and thermal
mapping.sup.14,15 can help provide a detailed understanding of
whole body and local SAR. Yet, rare as they may be compared to the
total number of MRI scans performed, RF burns do occur, a fraction
of which are reported to the FDA..sup.16 In these cases, at least,
a failure in scanner SAR monitoring has occurred.
[0009] Unfortunately, investigating whether the scanner is
operating safely within SAR guidelines by means that are
independent of the scanner, if performed at all, is not
easy..sup.17 The accuracy of scanner SAR estimates is also
questionable in light of discrepancies with thermally-derived SAR
measurements,.sup.17,18 especially during MRI safety-testing of
interventional devices.sup.3, 18-20 and the lack of correlation
between subjective heat perception by patients and scanner
SAR..sup.21
[0010] Setting precise SAR exposure levels for investigators
testing devices or MRI methods, or for evaluating SAR in individual
burn cases,.sup.22 requires accurate and independent measurement
tools. This starts with accurate measurements of the total power
deposited and requires a reliable RF power meter. The RF power
monitors built into the MRI scanner are usually attached to the RF
power amplifier output. However, measuring the power delivered to
the body is complicated by losses in the RF transmission chain,
including the cables, switches, the quadrature-hybrid (Q-hybrid)
and the MRI coil..sup.23,24 These losses can vary over time, but
are not routinely monitored.
[0011] Moreover, as we now report, the very high dynamic range
(typically >20 dB) (DR=peak-to-average power ratio) of RF
transmit pulses, and MRI duty cycles that span orders-of-magnitude,
are beyond the capabilities of available commercial power
meters..sup.25 These meters are adequate for pulse sequences with
short repetition periods (TR) and consistent high power levels.
However, they do not give accurate results for sequences using
mixtures of high and low amplitudes or modulations, or long TR.
There thus remains a need for improved systems and methods for
measuring high dynamic range, low duty cycle (typically <1%) RF
power.
SUMMARY
[0012] A device with at least one channel for measuring high
dynamic range, radio frequency (RF) power levels over broad-ranging
duty cycles according to an embodiment of the current invention
includes a power sensor circuit comprising at least one logarithmic
amplifier; at least one directional RF coupler electrically
connected to the at least one power sensor; at least one RF
attenuator electrically connected to the at least one RF coupler;
and at least one sampling circuit electrically connected to the at
least one RF attenuator and the at least one RF coupler. The at
least one sampling circuit performs analog-to-digital conversion of
electrical signals received to provide digitals signals for
measuring the RF power level in the at least one channel.
[0013] A method of monitoring RF exposure proximate an RF emitting
device according to an embodiment of the current invention includes
connecting an RF monitoring system according to an embodiment of
the current invention to an RF power source of said RF emitting
device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Further objectives and advantages will become apparent from
a consideration of the description, drawings, and examples.
[0015] FIG. 1 is a schematic illustration of the Philips Achieva RF
power delivery chain provided to facilitate a description of some
embodiments of the current invention. Shown are the RF amplifier
(RF AMP), measured cable attenuation factors (A, B, C), the filter
box penetrating the scanner's Faraday cage, the transmit switch
(TRSW) and the Q-hybrid.
[0016] FIG. 2A provides a circuit diagram of a digital power sensor
circuit (PSC) based on the AD8310 IC according to an embodiment of
the current invention. FIG. 2B shows an example of one of the three
power profiling measurement modules (PPMM) showing the directional
coupler and PSCs according to an embodiment of the current
invention.
[0017] FIG. 3 is a schematic illustration of a device for measuring
high dynamic range, low duty cycle radio frequency (RF) power
levels according to an embodiment of the current invention.
[0018] FIG. 4A is a schematic illustration of a bench setup for
calibrating the Power Sensor Circuit (PSC) units as well as testing
dynamic range and linear performance according to an embodiment of
the current invention. FIG. 4B shows a calibration curve, PSC vs.
Ladybug meter showing linearity. FIG. 4C shows PSC 90 dB dynamic
range.
[0019] FIG. 5 is a schematic illustration of a resonant circuit for
an MRI coil producing a certain transverse RF magnetic field, B1,
to facilitate an explanation of some concepts of the current
invention. Rc is the coil resistance and Rs is the resistance
reflected into the coil circuit by the imaging subject load. B1 is
proportional to current I in the coil and Power loss=I.sup.2.
(Rc+Rs), where R.sub.c is the coil resistance and R.sub.s is the
sample resistance at the RF, and I is the coil current. Shown also
are pickup loops used by the scanner to monitor the B1 RF field
produced by the coil.
[0020] FIGS. 6A-6C show real-time profile of a .about.7 ms
asymmetric, slice selective RF pulse in the Philips scanner. FIG.
6A shows scanner B1 linear pulse envelope simulator. FIG. 6B shows
recorded time window of RF power in real time for long RF pulses.
FIG. 6C shows a single pulse (logarithmic scale).
[0021] FIGS. 7A-7C show results for some measurements according to
an embodiment of the current invention. FIG. 7A shows Philips
Medical System's 3 Tesla (T) Achieva amplifier output power
measurements using the PPMM (solid circles), and the scanner power
monitoring unit (open diamonds, reported in the log file), vs.
scanner average predicted power. Power data are from 11 human
volunteers, plus four measurements from mineral oil bottles
(circled), using short (1 ms) and long (7 ms) slice-selective
pulses. The line represents identity. FIG. 7B shows total power
deposited in the body measured by the PPMM, as a function of BMI.
Lines of best fit correspond to long and short pulses. Also shown
is the PPMM-measured whole-body SAR for (FIG. 7C) short and (FIG.
7D) long pulses, as compared to the scanner predicted SAR from
scanner log files (horizontal lines).
[0022] FIGS. 8A-8B show measured SAR for 6 volunteers on the
Siemens Medical System's 3 T system (solid dots) compared to
scanner reported values from (8A) the log file and (8B) the scanner
console.
DETAILED DESCRIPTION
[0023] Some embodiments of the current invention are discussed in
detail below. In describing embodiments, specific terminology is
employed for the sake of clarity. However, the invention is not
intended to be limited to the specific terminology so selected. A
person skilled in the relevant art will recognize that other
equivalent components can be employed and other methods developed
without departing from the broad concepts of the current invention.
All references cited anywhere in this specification, including the
Background and Detailed Description sections, are incorporated by
reference as if each had been individually incorporated.
[0024] Some embodiments of the current invention provide a high-DR,
MRI-compatible, power profiling system for measuring and recording
RF power over a wide range of MRI scan conditions. The system is
broadband up to 400 MHz, can be used to sample power for both local
and whole-body power flow and, unlike commercial meters, has six
channels and a buffer size suitable for monitoring power at
multiple locations over extended time periods. We provide some
examples of its application to real-time RF power monitoring in
human whole-body MRI studies of volunteers performed in commercial
Philips Medical Systems' (Best, The Netherlands) and Siemens
Medical Solutions' (Malvern, Pa.) 3 T MRI scanners. We show that
the power deposited and the body-average SAR,.sup.1, 2 often vary
considerably from the scanners' own estimates.
RF Power Measurement
[0025] In an example, the losses in the RF power chain of a Philips
3 T Achieva 3 T scanner.sup.26 were first characterized using a
4395A Agilent Technologies (Santa Clara, Calif.) network analyzer
by measuring the attenuation in each stage in accordance with the
schematic in FIG. 1. Measured losses in these components show that
the power output at the Q-hybrid (points D, E, FIG. 1) is only
about 59% of the power out of the RF amplifier (point A). (See, for
example, U.S. Pat. Pub. No. 2011/0148411; U.S. application Ser. No.
12/677,097 assigned to the same assignee as the current
application, the entire content of which is incorporated herein by
reference.)
[0026] To measure the pulse power during MRI, we first tried
commercial inline power meters. Bird 5014 and Bird 5010b (Bird
Technologies, Solon, Ohio) did not work correctly for peak/average
power ratios greater than ten. Even when operating the scanner at
minimum TRs and low RF field intensity (B.sub.1), measurements were
unstable and irreproducible.
[0027] We next used a Ladybug Technologies LLC (Santa Rosa,
Calif.), LB480A power profiling meter in combination with .sub.50
dB dual directional couplers to measure forward and reverse power
at the outputs of the power amplifier and the Q-hybrid during MRI.
The Ladybug meter sampled the pulse profile at 10 .mu.s intervals
and stored results for power calculations. While this yielded
accurate measurements on four volunteers,.sup.27 the use of USB
cables from Ladybug to the computer necessitated a person inside
the scanner room. Moreover, the Ladybug meter did not have
sufficient channels for monitoring the forward and reverse power at
the three locations of interest simultaneously (A, D, and E in FIG.
1, location A). In addition, its small buffer size (.ltoreq.1 sec)
was inadequate for providing continuous measurements of power for
many MRI sequences with long TRs over the several cycles needed for
accurately measuring time-averaged power, thus rendering real-time
measurements impractical.
[0028] We therefore built a 6-channel scanner-independent power
monitoring system according to an embodiment of the current
invention. An embodiment has six power sensor circuits (PSC), (FIG.
2A) assembled from AD8310 logarithmic amplifier IC's (Analog
Devices, Norwood Mass.). At each of three locations (RF amplifier
output, two Q-hybrid outputs), a power profiling measurement module
(PPMM) having a 50 dB directional coupler (Werlatone Inc.,
Patterson N.Y.) connected to two PSCs, one to its forward channel
and one to its reverse channel (FIG. 2B), was deployed. A 10 dB
attenuator was added to the forward channels to allow measurements
of up to 50 kW of peak power. The design DR was from 17 dBm (nearly
the maximum input power of the AD8310) to -80 dBm over the desired
frequency range 1-440 MHz. Each PSC is powered by a rechargeable
lithium ion, non-magnetic 4V battery (PowerStream, Orem Utah) and
can operate continuously for at least 10 hours before recharging.
The video bandwidth of the ICs was set to 112 KHz using a 470 PF
capacitor (FIG. 2A).
[0029] Although this embodiment describes three circuits, such as
the one illustrated in the embodiment of FIGS. 2A and 2B, one, two,
three or more such circuits could be used in various embodiments of
the current invention.
[0030] The outputs of each of the six PSCs are simultaneously
sampled in differential input mode at 200 kHz by a 16 bit USB-6251
National Instruments (Austin, Tex.) data acquisition system
controlled by a laptop computer that also stores the power
measurement data. The 5 .mu.s sampling resolution accurately
captures the MRI RF pulse modulation whose time resolution in the
Philips scanner was about 6.4 .mu.s. A MATLAB (The Mathworks,
Natick, Mass.) program was written to read the saved voltage files,
convert them to power profiles using the linear calibration curves
for each channel, and to calculate average power values for all
experiments. A schematic of the system configured to monitor RF
power flow is shown in FIG. 3. The (low frequency) power profiling
lines from the PPMMs attached to the quad hybrid outputs are fed
through the scanner room's connection panel. The lines from the
PPMM connected to the RF power amplifier were wound around ferrite
cores to prevent RF interference.
[0031] Each PPMM was bench calibrated for the operational
frequencies of the Philips 3 T Achieva scanner and a Siemens 3 T
Trio scanner (127.8 MHz and 123 MHz, respectively) using the setup
shown in FIG. 4A. The calibration was performed against the LB480A
meter using a 10 dBm frequency synthesizer whose output was
connected to a 0-100 dB variable attenuator to vary the input power
level. The PSC voltage-to-logarithmic power was measured over a 70
dB range (limited by the LB480A unit's operational dynamic range)
and was highly linear as shown in FIG. 4. FIG. 4B. The slopes of
the calibration curves were about 0.24 V/10 dBm. The net sampling
resolution of the A/D was set to 0.004 dBm. After calibration the
full DR was tested over a range of 90 dB as shown in FIG. 4C and
exhibited a maximum deviation of 0.8 dBm from linearity at -80 dBm.
The total insertion loss of the monitoring system PPMMs at 128 MHz
was 0.1 dB or about 2%.
RF Power Deposition
[0032] FIG. 5 shows a schematic resonant circuit for an MRI coil
producing a transmit RF field, B1, proportional to the current, I,
in the coil. The power loss in the circuit is the sum of the coil
and subject losses in resistive loads Rc and Rs, respectively. The
pickup loops, are fixed by the manufacturer inside the RF body
coil. They are used by the scanner to monitor and set the initial
value of the RF field produced by the coil during set-up. The power
loss in the coil P.sub.coil, is measured as the net power flow at
the output of the Q-hybrid with a lossless sample placed in the
coil. The lossless sample is a 1 liter bottle of mineral oil whose
RF dielectric constant, conductivity and size are
orders-of-magnitude lower than those of the body..sup.28,29 This
was verified by measuring P.sub.coil with additional mineral oil
sample volumes of 2 liters and 3 liters; no significant change in
power absorption was observed. The desired B1, and therefore the
current I required to produce it, is approximately constant,
independent of the subject being imaged..sup.30,31 Therefore the
coil power dissipation, P.sub.coil, is constant for a given pulse
sequence, independent of the subject. The power deposited in the
subject is then P.sub.subject=P.sub.total-P.sub.coil, where
P.sub.total is the total power dissipated in the coil plus the
subject measured at the Q-hybrid. Note that larger subjects have
greater P.sub.total but the same P.sub.coil.
[0033] To measure the RF power deposited in human subjects during
MRI, the power monitoring system was connected to the output of the
RF power amplifier and the two outputs of the Q-hybrid before the
scan. Eleven healthy volunteers (9 men, 2 women; age 22-65 yrs)
were recruited and provided informed consent for this study
approved by The Johns Hopkins Institutional Review Board on Human
Investigation. Subjects were positioned in the Philips 3T scanner
and the scanner's automated scan preparation sequence initiated.
Volunteers were landmarked at the xiphoid, placed at the isocenter
of the scanner and a transverse slice containing both the heart and
liver was targeted. A reference B1 RF field is first set based on
pickup coil sensors, followed by the scanner's MRI-based B1
optimization algorithm which sets the final flip-angle. The B1
optimization algorithm is based on a stimulated echo sequence
similar to the one described by Akoka et al..sup.32 where an
average signal projection is used, thus rendering the result stable
against local field variations. Two field-echo (FE) MRI sequences
with TR=50 ms and two different RF pulse shapes (a short 1 ms
asymmetric two lobe pulse and a long 7 ms "Spredrex" pulse.sup.33)
were used.
[0034] The total scanner time per subject--including entry,
positioning, and egress from the scanner--was 10-15 min.
[0035] The delivered RF power reported by the scanner, as well as
the measured power output, was recorded. The subject was replaced
by the mineral oil phantom and the pulse sequence repeated to
produce the same B1 detected by the pickup coil. Scanner SAR and
power were again recorded, along with the power measured by our
power monitoring system. Body-average SAR was taken as the power
deposited divided by the subject's weight, in accordance with the
standard definition..sup.1,2
[0036] The same protocol was repeated on six of the volunteers
(men, age 23-66 yrs) in a Siemens 3T Trio scanner. The FE sequence
used the scanner's default 2 ms RF sinc pulse with one side lobe.
The scanner's console SAR differed from the value reported in its
log file, so both values were recorded.
[0037] All power values measured by our power monitoring system
were calculated by averaging instantaneous power over a 0.5 s time
window (10 pulses for FE pulse sequence).
Results
[0038] MRI experiments showed no noticeable interference or image
degradation with the PPMMs connected. Connecting the PPMMs did not
increase noise, as was confirmed by noise scans acquired with the
RF and gradients turned off.
[0039] FIG. 6 exemplifies the 6-channel real-time recordings of an
asymmetric, multi-lobe, slice selective RF pulse on the Philips
scanner with a subject present. The detailed instantaneous
recording of RF power is shown for Spredrex pulses on a logarithmic
scale..sup.33
[0040] The results for the forward power delivered to the Philips
body MRI coil and body average SAR for all subjects in the Philips
scanner are plotted in FIG. 7 as a function of the power reported
by the scanner, the patient weight, and the body mass index (BMI).
FIG. 7A shows that the scanner-reported power at the RF amplifier's
output agrees with our PPMM system results to within 6% for short
pulses (.about.1 ms). This is not true for longer pulses (.about.7
ms), where the scanner's RF power monitoring fails when compared to
the PPMM system that had been calibrated over the full DR and
duty-cycles used for MRI. For all volunteers, the power delivered
at the output of the Philips quadrature hybrid (Q-hybrid) is
56.5.+-.2.5% of the output of the amplifier. This figure is
consistent with the 58.5% predicted from the measured losses in the
Philips RF chain plus the measured insertion losses in the power
monitoring modules. For the 50 ms TR, the average power dissipated
in the coil is 8.8.+-.0.6 W for the short pulses and 11.1.+-.0.8 W
for the long pulses, independent of the size of the mineral oil
bottle (1-3 liter).
[0041] The Philips Achieva scanner initially establishes a B1 that
is the same for all samples using pickup loops. It is worth noting
that the final MRI optimization yielded a B1 that was, on average,
within 5% of the initial pickup loop B1 in all samples, from small
to large human subjects as well as in the mineral oil bottles. This
result supports the assumption that the current I required to
produce a desired MRI flip angle across the slice projection is
essentially independent of sample size, and that the power
dissipation in the RF coil always equals the power dissipation with
the mineral oil sample to a good approximation.
[0042] FIG. 7B shows that the measured deposited power varies
linearly with BMI with a correlation coefficient R.sup.2=0.8 (0.7)
for short (long) RF pulses. FIG. 7C and FIG. 7D show that the
scanner almost always overestimates body-average SAR. The scanner
overestimated SAR by up to 78% for short pulses and 123% for long
pulses when compared to values obtained from our PPMM direct power
determination and subject weights.
[0043] FIG. 8 shows the calculated SAR values from the real-time
power monitor versus the scanner reported values for 3T Siemens
scanner. The power delivered at the output of the Q-hybrid is
90.+-.2% of the power measured at the RF amplifier output. SAR
values listed in the Siemens log file differ from those reported at
the console: Siemens does not state which values they use. In any
case, as with the Philips scanner, the Siemens scanner
overestimated SAR. The Siemens scanner log overestimated SAR by up
to 103% while the console values were up to 71% above the actual
measured SAR.
DISCUSSION
[0044] Some embodiments of the current invention address the
problem of providing accurate real-time measurements of the RF
power delivered to the body, which is inadequately served by
existing technology. Specifically, we found that two commonly
available commercial RF power meters are unsuitable for the full
range of DRs, duty cycles and pulse types encountered in MRI. This
was further underscored by differences and errors in power
monitoring for short and long RF pulses in the Philips scanner. We
therefore developed a real-time, multi-channel power monitoring
system according to an embodiment of the current invention that is
suitable for a full range of MRI RF pulses and sequences operating
over a frequency range that will accommodate scanners with fields
up to 10 T..sup.34
[0045] The accuracy of measurements provided by our power
monitoring system was independently validated three ways: 1) on the
bench using the Ladybug power meter (FIG. 4); 2) using the 3T
scanner's power monitoring unit at the output of the amplifier for
high-power, short RF pulses (FIG. 7A); and 3) by measuring the
losses in the 3T scanner's RF chain using our power monitor and
comparing the results with independent measurements made with a
network analyzer.
[0046] Our new power monitoring system was used to determine the
true power deposited and the body-average SAR delivered to adult
volunteers in two clinical 3 T MRI systems. The results showed that
the scanners almost always overestimate the body-average SAR as
compared to the actual power deposited. The overestimates were as
much as 120% and 100%, respectively, in the Philips and Siemens 3T
systems studied here (FIG. 7C, 7D; FIG. 8). Unfortunately, the
exact details of the manufacturer's SAR modeling are proprietary,
precluding the identification of specific causes for the
differences. Nevertheless, the data in FIG. 7 suggest Philips' use
of a worst-case estimate that is independent of the subject
loading, while Siemens' model evidently depends on the subject's
weight (FIG. 8). Although the evaluations were performed on the
scanner's whole-body coils with sample-dominant losses, application
of the power monitoring system is not limited by coil geometry, and
similar measurements could be performed on other vendors' scanners
and other coil sets, including multi-transmit systems at various
field strengths..sup.34
[0047] The power monitoring system and protocol presented here
provide measures of the total power deposited in the body during
MRI, or the body average SAR defined as the total power divided by
the subject's weight..sup.1, 2 Local SAR exposure, such as peak 1-g
or 10-g averages, are also important for safety compliance..sup.1,2
At present, these must be obtained by numerical electromagnetic
modeling,.sup.11-14, 35-37 from which ratios of the peak local SAR
to the total power can be derived. These are, however, anatomy
dependent. In practice, the total deposited power may be used in
conjunction with numerical electromagnetic models to provide
estimated local SAR values. .sup.17, 35-40
[0048] In the Philips scanner, because of losses in the cables, RF
coil and other transmit chain components, the power reaching the
imaging subject in the Philips scanner was less than half the power
supplied by the RF transmitter. The smaller power loss for the
Siemens scanner indicates the use of lower loss components.
Moreover, both scanners' whole-body SAR estimates reported to the
scanner operator seem conservatively overstated. While this may
provide an extra safety margin for RF exposure, it nevertheless
means that scanner SAR values are not reliable for specifying RF
exposure when testing the MRI safety of peripheral, implanted and
interventional devices..sup.3, 20 The overestimate may also limit
high-SAR pulse sequences, forcing unnecessary reductions in duty
cycle or pulse power that increase scan time and/or compromise
efficiency.
[0049] Some embodiments of the current invention can provide a
versatile approach to accurately measure, in real-time, the total
RF power deposition during MRI, independent of the scanner. We have
used our real-time power monitoring system to demonstrate
deficiencies in commercial scanner reported RF SAR values in some
examples. Some embodiments can be used to monitor regulatory
compliance, SAR dosimetry, evaluation of scanner function following
burn injuries and for setting RF exposure levels during device
safety testing. In addition, applications of the current invention
are not limited to MRI, but can be used for measuring RF power in
other applications including radar, medical RF diathermy, RF
ablation systems, and RF telecommunications systems.
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[0090] The embodiments illustrated and discussed in this
specification are intended only to teach those skilled in the art
how to make and use the invention. In describing embodiments of the
invention, specific terminology is employed for the sake of
clarity. However, the invention is not intended to be limited to
the specific terminology so selected. The above-described
embodiments of the invention may be modified or varied, without
departing from the invention, as appreciated by those skilled in
the art in light of the above teachings. It is therefore to be
understood that, within the scope of the claims and their
equivalents, the invention may be practiced otherwise than as
specifically described.
* * * * *
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